1. Field of the Invention
The present invention relates to a multimode optical fiber and its manufacturing method, and more particularly to a multimode optical fiber for high data rate LAN (Local Area Network), which improves transmission properties for high data rate LAN by eliminating defects in a core region and also suggests a criteria for minimum transmission performance required for gigabit level optical transmission, and its manufacturing method.
2. Description of the Related Art
As the number of Internet users is increased, more transmission capacity is needed for stable communication service, so more and more interests are taken in 1- or 10-gigabit level LAN system using a multimode optical fiber as a transmission line, which gives better transmission performance together with a relatively lower maintenance cost rather than the conventional system. However, in spite of such expectation, the existing gigabit-level LAN system is not easy to cope with subscriber's demand for bandwidth, which is explosively increased, due to its structural disadvantage that it generally uses LEDs as a light source. Accordingly, there is a need for a transmission system which may use a laser diode as a light source capable of receiving more transmission capacity.
The transmission system using a laser diode as a light source may be efficiently used in a system of more than 10-gigabit level. However, more preferably, the system should be specified to meet user's demand at a low cost through a suitable compromise between a system configuration cost and a system performance, which are essential factors of LAN. VCSEL (Vertical Cavity Surface Emission Laser) diode and Fabry-Perot LD (Laser Diode) may be used at a relatively low cost among the existing laser diodes supporting a data rate over 1 gigabit.
However, though advantageously supporting a high data rate communication over a gigabit level, the laser diode causes several problems when it is used together with a multimode optical fiber, differently from LED. Representatively, since a laser diode is configured to irradiate a light only to a partial area of a core center of an optical fiber when the gigabit-level system is used, fine defects and irregularity in the core may sensitively transform an output signal, thereby exerting serious effects on performance deterioration of the system.
Thus, it is understood that research for an optical fiber manufacturing process which may eliminate such drawbacks in the core region is most essential to configure a system which may realize high data rate optical transmission regardless of the kind of a light source.
FIG. 1 shows MCVD (Modified Chemical Vapor Deposition), which is a representative optical fiber manufacturing method currently used. Sections (a), (b) and (c) in FIG. 1 respectively show a deposition process, a collapse process and a drawing process, which configure MCVD in order.
Referring to the section (a) of FIG. 1, in the deposition process, deposition gas such as SiCl4, GeCl4, POCl3, He and O2 is injected into a quartz tube 10 which generally rotates at 20 to 120 rpm, and a heat source 5 is slowly moved along an axial direction of the quartz tube 10 to heat outside of the tube, thereby forming a deposition layer 12 composed of a core and a clad.
More specifically, the deposition gas injected and flowed in the quartz tube 10 is heated up to a reaction temperature at a position adjacent to the heat source 5. At this time, due to thermal oxidization, a fine silica particle layer 11 is generated on an inner wall of the tube positioned in front of the heat source 5 and having a relatively low temperature, and the fine silica particle layer 11 is sintered to form the core/clad deposition layer 12. Whenever the heat source 5 moves once along the entire length of the quartz tube 10, one layer of the particle deposition layer is obtained. Thus, if such procedure is repeated several ten times and constitution of the deposition is changed to give a desired refractive index distribution for each layer, the clad and core deposition layers 12 are subsequently formed in the quartz tube 10.
After the deposition process is completed, the collapse process as shown in the section (b) of FIG. 1 follows. That is to say, if the outside of the quartz tube in which the clad and core deposition layers 12 are formed is heated over a deposition temperature (e.g., at 2000 to 2300° C.) by means of the heat source 5 moving in an axial direction, viscous flow is generated in the quartz tube 10, so inner and outer diameters of the tube are gradually decreased due to the difference of surface tension and pressure between the inner and outer walls. If this procedure is repeated several times, a gap G in the quartz tube 10 is substantially filled up, thereby making an optical fiber preform of a quartz rod shape. For the optical fiber preform having experienced the collapse process, the drawing process as shown in the section (c) of FIG. 1 is accomplished to resultantly obtain an optical fiber.
Generally, GeO2 is doped as an additive to increase a refractive index while the core layer is formed in the deposition process. This additive is volatilized during the collapse process which is progressed at a higher temperature than the deposition process, as shown in the following reaction formula 1. In the reaction formula 1, (s) and (g) respectively indicate a solid state and a gas state of substance.
                              Ge          ⁢                                          ⁢                                    O              2                        ⁡                          (              s              )                                      ↔                              Ge            ⁢                                                  ⁢                          O              ⁡                              (                g                )                                              +                                    1              2                        ⁢                          O              2                                                          Reaction        ⁢                                  ⁢        Formula        ⁢                                  ⁢        1            
Due to the reaction like the reaction formula 1, GeO2 concentration is decreased on the surface of the deposition layer of the core center, and an optical fiber preform finally made has a refractive index distribution with an index dip as shown in FIG. 2. In addition, GeO gas generated by the reaction is partially condensed again into GeO2 in front of the moving heat source 5. Thus, as the heat source 5 is moved, internal diffusion of GeO2 is activated again, so it probably cause an index peak that a refractive index is increased again at the core center as shown in FIG. 3.
Such index dip and index peak, and resultant axial irregularity of refractive index, significantly reduce a bandwidth of a multimode optical fiber, thereby deteriorating optical characteristics thereof. Thus, the index dip and peak are a problem which should be solved, particularly in the process of making a multimode optical fiber for a gigabit level transmission system in which an optical signal is irradiated only to a part of the core region. It is because deterioration of optical transmission characteristics is inevitable if the transformation of refractive index profile caused by volatilization or re-condensation of additives generated in the collapse process is not eliminated, even though the refractive index is ideally controlled in the deposition process.
In order to minimize the change of refractive index due to volatilization of GeO2 during the collapse process, a method for compensating volatilized GeO2 by injecting O2 and GeCl4 into a quartz tube just before the final collapse process as shown in the following reaction formula 2 has been proposed in U.S. Pat. No. 4,165,224 and No. 4,304,581 and by Akamatsu et al. (1977, Appl. Phsy. Lett, 31. 515˜517).GeCl4(g)+O2(g)GeO2(s)+2Cl2(g)  Reaction Formula 2
In addition, U.S. Pat. No. 4,921,516 revealed that an overdoping process according to the reaction formula 2 should be conducted at a temperature lower than the collapse process so that a deposition layer exists in a colloidal state, and a thickness of the deposition layer should be gradually decreased in a forwarding direction of a heat source during the final collapse process in order to improve compensation effects.
U.S. Pat. No. 4,657,575 discloses that Al2O3 is used as an additive instead of GeO2. According to this document, when Al2O3 with a melting point of 2045° C. is used for controlling a refractive index of the optical fiber, diffusion of the additive out of the core is suppressed during the collapse process rather than the case of using GeO2 with a melting point of 1086° C., thereby decreasing an index dip, compared with the conventional processes.
Besides the aforementioned methods, a technique for making a final optical fiber preform after removing a volatilized portion of GeO2 by etching just before the final process among the collapse processes has been proposed. At this time, the etching process may use a reaction gas such as HF (Hopland, 1978, Electron. Lett., 14, 757˜759) or gaseous fluoric compound (Liegois et al., 1982, Non-Cryst. Solids. 117, 247˜250; Schneider et al. 1982, Conf. Proc. Eur. Conf. Opt. Fibre Commun. 8th., 36˜40). U.S. Pat. No. 4,793,843 discloses that an amount of fluorine per a unit area may be increased and an etching effect may also be improved by using a fluoric compound such as C2F6, C3F8 and n-C4F10 together with O2 in the etching process. However, this technique conducts several times of the etching process separatively just before the final collapse process, so a time gap exists between N−1th and Nth collapse steps and thus GeO2 is volatilized during the etching process. In addition, since there is a limit in decreasing an inner diameter of the tube just before the final collapse process due to the etching process, a volatilizing area of GeO2 is still large, so it is substantially not easy to effectively eliminate the index dip.
Though such various techniques are proposed, there is realistically not obtained a multimode optical fiber whose drawbacks are sufficiently eliminated to be suitable for a gigabit level high data rate transmission system, so there is still a need for a new method capable of more effectively eliminating drawbacks of the core center such as an index dip and an index peak.
Meanwhile, in order to use a multimode optical fiber for a gigabit level high data rate LAN, a transmission protocol which provides an optimal transmission performance regardless of the kind of a light source should be suggested.
As a conventional manner of indicating the transmission characteristics of a multimode optical fiber, there is a Restricted Mode Launching Bandwidth (RMLB) regulated by FOTP-204. However, the kinds of light sources to which RMLB may be applied are restricted, so, if a light source or a light-exciting condition is changed in the actual use, a critical error may arise since RMLB does not satisfy an actually demanded bandwidth.
FOTP-220 regulates a method for measuring DMD (Differential Mode Delay) having an improved accuracy in comparison with RMLB. FOTP-220 is known as a method which may evaluate transmission characteristics of a multimode optical fiber more accurately regardless of the used light source than any other existing method. A transmission characteristic evaluation criterion of a multimode optical fiber according to FOTP-220 is specified in TIA-492AAAC, which however has limitations that it is restrictively applied to a multimode optical fiber with a core diameter of 50 μm, a transmission distance up to 300 m and an application wavelength of 850 nm, among 10-gigabit level optical fibers.
As mentioned above, transmission characteristics of a multimode optical fiber, which may be applied to all of 1-gigabit level and 10-gigabit level high data rate optical transmission systems and may be used at both 850 nm and 1300 nm regardless of the kind of a light source, has not been proposed in the past, so there is a need for its alternative.